A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including a photoconversion device, for example a photogate, a photoconductor, or a photodiode, for generating and accumulating photo-generated charge in a portion of the substrate. A readout circuit is provided within each pixel cell and includes at least an output transistor, which receives photogenerated charges from the photosensor through a doped diffusion region and produces an output signal which is periodically read-out through a pixel access transistor. The imager may optionally include a transistor for transferring charge from the photoconversion device to the diffusion region or the diffusion region may be directly connected to or be part of the photoconversion device. A transistor is also typically provided for resetting the diffusion region to a predetermined charge level before it receives the photoconverted charges.
An imager circuit having an array of pixel cells often has an associated color filter, such as a color filter arranged in a Bayer pattern for discerning differing wavelengths of light in different pixel cells.
Exemplary CMOS imaging circuits, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. No. 6,140,630 to Rhodes, U.S. Pat. No. 6,376,868 to Rhodes, U.S. Pat. No. 6,310,366 to Rhodes et al., U.S. Pat. No. 6,326,652 to Rhodes, U.S. Pat. No. 6,204,524 to Rhodes, and U.S. Pat. No. 6,333,205 to Rhodes. The disclosures of each of the forgoing patents are herein incorporated by reference in their entirety. 6,333,205 to Rhodes. The disclosures of each of the forgoing patents are herein incorporated by reference in their entirety.
FIG. 1 illustrates a block diagram for a CMOS imager device 308 having a pixel array 200, which contains a plurality of color pixels arranged in a predetermined number of columns and rows. The pixels of each row in array 200 are all turned on at the same time by a row select line, and the pixels of each column are selectively output by a column select line. The row lines are selectively activated by the row driver 210 in response to row address decoder 220 and the column select lines are selectively activated by the column driver 260 in response to column address decoder 270. Thus, a row and column address is provided for each pixel. The imager is operated by the timing and control circuit 250 which controls address decoders 220, 270 for selecting the appropriate row and column lines for pixel operation and readout. Row and column driver circuitry 210, 260 apply driving voltage to the drive transistors for selected row and column lines. The column lines are coupled to sample and hold circuits 261, 262, which sample and hold a reset voltage Vrst and a signal voltage Vsig for each pixel. A differential signal Vrst−Vsig is produced for each pixel. The differential signal is amplified and digitized by analog to digital converter 275 and fed to an image processor 280, which produces an output image signal from the digitized pixel signals.
The gain characteristics of the pixel signals are an important factor for proper operation of the imager array and subsequent processing circuitry. Different colored pixels have different light response gains. In addition, the analog signal processing circuitry and analog to digital conversion must be balanced and calibrated to remove aberrations in the gain characteristics for the different colors. Conventional methods require a great deal of processing to adjust and test the pixel signal gain characteristics of the various pixel signal processing circuits. The adjustment of gain characteristics during processing is complicated and requires the occupation of valuable real estate on the chip. Testing and calibration of the gain characteristics requires the production of signal outputs under varying light or voltage conditions. Multiple measurements are required to obtain a single result.
Calibration of integration time and analog to digital conversion accuracy are examples of gain characteristics that are typically calibrated and optimized in an imager device. The integration time is the amount of time that the pixel is receiving light photons, converting the photons to a charge and accumulating the charge, before the charge is stored or read out. The integration (i.e. exposure) time can be reduced to optimize exposure to the dynamic range of the pixel and to control potential blooming issues. Blooming can occur when too many photons strike a particular pixel cell and generate charges that overflow into adjacent pixels, causing the adjacent pixels to incorrectly sense the image. However, some colors saturate more quickly than others and different arrays have differing saturation points and gain characteristics. Pixel sensor cells will vary in their saturation point based on the wavelength of the particular color being absorbed, light intensity, gain characteristics and integration time. Since typically three color sensors (red, blue, green) are needed to correctly sense an image, it is difficult to customize and synchronize the integration times of the different colors having different saturation points under variable light intensity conditions to control blooming and saturation.
The accuracy of the analog to digital conversion process is another gain characteristic which is typically calibrated during processing. FIG. 1 shows analog to digital converter (ADC) 275, which during operation converts analog signals from the sample/hold circuits 261, 262 to digital signals that are fed to the image processor 280, which outputs the image signal from the digitized pixel signals. The ADC 275 must be calibrated so that analog to digital conversion is accurate and optimal during operation of the imager. Typically, ADC calibration requires a stable external voltage source. The stable external voltage source is used to apply differing and known voltages to the converter and the resulting digital output signal is read out and compared to an expected digital output signal. Each known voltage is applied separately. Conventional technology requires multiple measurements to obtain ADC calibration information which is undesirable. There is a need for a faster and more efficient method of determining light intensity and performing ADC calibration on the pixel array.